1. Field of the Invention
The present invention relates to an image inputting apparatus which uses a solid state imaging device, and more particularly to an improvement in image inputting apparatuses equipped with image shifting mechanisms for taking a plurality of images with different resolutions.
2. Description of the Related Art
Solid state imaging devices such as CCDs have long found widespread application as imaging devices for use in image inputting apparatuses. In principle, resolutions of solid state imaging devices are essentially determined by the number of pixels arranged in the image pickup surfaces. Accordingly, use of more pixels results in higher resolutions. However, some limits presently exist on the increased number of pixels from a technical standpoint and in terms of cost. Some techniques have already been suggested, which allow images to be taken at relatively higher resolutions through the use of solid state imaging devices composed of limited numbers of pixels, such as methods which involve the use of image shifting mechanisms, as disclosed in Japanese Unexamined Patent Publication JP-A 60-54576 (1985). More recently, there also have been suggested image inputting apparatuses equipped with image shifting mechanisms, which are designed to take two or more different images: an image of the resolution increased by image shifting and another image of the resolution characteristic to the CCD used. The related art is also described in, for example, Japanese Unexamined Patent Publication JP-A 3-231589 (1991) and Japanese Unexamined Patent Publication JP-A 3-226078 (1991).
FIG. 17 illustrates an example of the configuration of an image inputting apparatus which allows two images to be taken at different resolutions. Arranged along an optical axis Z are an optical system 1, a refracting plate 2, a spatial filter 3 and a solid state imaging device 4. The refracting plate 2 may be displaced by an actuator such as a piezoelectric device 5, from a position where the surface is perpendicular to the optical axis Z indicated by the solid line to a position where it is tilted with respect to the optical axis Z which is indicated by the broken line. The piezoelectric device 5 is driven by a piezoelectric-device driving circuit 6. Images taken by the solid state imaging device 4 are converted to electric signals which are outputted as image information through an image-processing circuit 7. A control circuit 8 controls the piezoelectric-device driving circuit 6 on the basis of the image information from the image-processing circuit 7 so that the optical axis Z is shifted to the optical path Z' indicated by the broken line by displacement of the refracting plate 2 by the piezoelectric device 5. The amount of this displacement of the optical path by image shifting is set to be half the pixel pitch of the solid state imaging device 4, for example. A detailed explanation of this type of image shifting mechanism is given in Japanese Unexamined Patent Publication JP-A 60-54576 mentioned above.
The image inputting apparatus illustrated in FIG. 17 is designed to input an image with a first resolution, that is, the characteristic resolution (hereunder may be referred to as "normal resolution") determined by the pixel number of the solid state imaging device 4 in cases where image shifting is not performed. The control circuit 8, upon receipt of a signal indicating input mode at the normal resolution from a controller (not shown), instructs the piezoelectric-device driving circuit 6 to suspend its operation to thereby stop the piezoelectric device 5 at a given position. The given position of the piezoelectric device 5 is the one at which the refracting plate 2 is not tilted and thus the optical axis Z is not shifted. The spatial filter 3 serves to cut off high spatial frequency components from an image incident from the subject which have moire-producing (at the next solid state imaging device 4) spatial frequencies over the Nyquist frequency. Output signals of an image incident upon the solid state imaging device 4 undergo A/D conversion, gamma correction, etc. through the image-processing circuit 7, and are obtained as signals of the image with the normal resolution. The image output means is a cathode ray tube (hereunder abbreviated to "CRT") when the image is outputted as a dynamic image, and an image memory or the like when it is outputted as a still image. In either case, the processing technique may be any well known one. Here, "dynamic image" means that which is formed of still images in time-series, typically images taken with video cameras or the like.
An explanation will now be given regarding the operation for inputting an image with a second resolution which is higher than the first resolution (hereunder may be reffered to as "higher resolution") through image shifting. The control circuit 8, upon receipt of a signal indicating a mode for inputting an image with the higher resolution from the controller (not shown), instructs the piezoelectric-device driving circuit 6 to perform an image shifting operation. According to the instructions, the piezoelectric-device driving circuit 6 drives the piezoelectric device 5 to displace the refracting plate 2 alternately to the position indicated by the solid line and to the position indicated by the broken line, and the image shifting operation is performed at the position indicated by the broken line to shift the optical axis Z to the optical path Z'. The image-processing circuit 7 subjects a plurality of images taken via image shifting to A/D conversion, gamma correction, etc., and an image with the second resolution which is higher than the first resolution is synthesized in an image memory. An image synthesis method is described in, for example, Japanese Unexamined Patent Publication JP-A 63-284980 (1988) in detail.
When an image with the normal resolution is inputted, the spatial filter 3 cuts off high spatial frequency components of the inputted image so as to remove a moire due to sampling of the image inputted by the solid state imaging device 4. Japanese Unexamined Patent Publication JP-A 4-236585 (1992) discloses the concept of optical low-pass filtering which enables both moire removal and increase in the resolution by making the transmitting characteristics of the spatial filter 3 variable in such a manner that spatial frequencies which allow transmission through the spatial filter 3 are set to be low when images with the normal resolution are inputted, and high when images with the higher resolution are inputted. Japanese Unexamined Patent Publication JP-A 3-226078 discloses a prior art technique which involves provision of two replaceable types of spatial filters, a low-cutoff spatial filter which is to be used when images with the normal resolution are inputted, and another high-cutoff spatial filter which is to be used when images with the higher resolution are inputted. In addition, Japanese Examined Patent Publication JP-B2 1-35550 (1989) discloses a method in which the spatial filter 3 is not used, and image shifting is utilized to input still images to acquire still images with the higher resolution, whereas when images with the normal resolution are inputted, the refracting plate 2 is vibrated at a high speed to remove moires, and the opening of the solid state imaging device 4 is temporarily extended to acquire images with the normal resolution.
Of the conventional techniques which accomplish both inputting of images with the higher resolution and moire removal, those methods such as the one disclosed in Japanese Unexamined Patent Publication JP-A 4-236585 which depend on the variable transmitting characteristics of the spatial filter 3, have a disadvantage in that the spatial filter 3 requires a complicated configuration, and this results in an increased manufacturing cost. In addition, the prior art techniques employing such a configuration as the one disclosed in Japanese Unexamined Patent Publication JP-A 3-226078 which contains two replaceable types of spatial filters 3, requires a mechanism for switching between the spatial filters 3, and this increases the size of the image inputting apparatus as well as the manufacturing cost. The method in which when a still image is inputted, image shifting is performed without using the spatial filter 3, and the refracting plate 2 is vibrated at a high speed so as to remove a moire when an image with the normal resolution is inputted, requires that the refracting plate 2 be driven at all times even when an image of the subject which does not produce a moire is inputted at the normal resolution, and this of course increases the power consumption; the method is particularly unsuitable when the solid state image inputting device is portable, because it results in accelerated exhaustion of the battery.